PARASITES OF ANIMALS. 55 
and is supposed to perform the office of excretory organs, 
analogous to the kidneys and liver of vertebrate animals. In 
the central region of the body there is a well developed repro- 
ductive system, both male and female organs being generally 
contained in the same individual worm; but in. some species 
the sexes are separate. All the rest of the body, around these 
organs, is composed of a rather firm and solid tissue, the or- 
gans not being contained in a distinct cavity. 
The species constituting this order are very numerous, be- 
longing to several distinct families and many genera. About 
400 species have been described. They are found in all. classes 
of vertebrate animals, and are numerous in their larval states 
in many invertebrates, especially in fresh-water snails. They 
particularly abound in fishes, frogs, and aquatic birds. Nearly 
all the species undergo very remarkable transformations, with 
“alternate generations, some of them passing different stages of 
their lives free in water, and then in two or more distinct an- 
imals, as parasites. So that the adult forms, found in the 
higher animals, are generally derived from larve swallowed 
in small mollusks, like the river snails, and perhaps in insects. 
WY. AcanrnocepHata. (Thorn-headed worms.) 
This order includes elongated, more or less cylindrical 
worms, usually with the body encircled by distinct transverse 
ridges and wrinkles, and which have at the anterior end a 
prominent, elongated or conical, retractile proboscis, covered 
with numerous recurved hooks, by means of which they at- 
tach themselves tothe mucous membrane in the intestine of the 
animals that they inhabit. The body contains a large cavity, 
in which the reproductive organs are contained. The sexes are 
distinct. The ovary, situated in‘the anterior part of the body, 
sets free large roundish masses of cells from which the ova are 
afterward developed, while these masses, or “cocoons,” are 
free in the abdominal cavity, each mass containing a large 
number of eggs, often several hundred. In the male the two 
testicles are attached to the lower closed end of the digestive 
sac, and are connected by ducts with the intromittent organ, 
which is a slender spiculum, enclosed in a sheath, situated at 
the posterior end of the body. 
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